the invention relates to a diagnostic device for detecting short circuits or line interruptions in an inductive sensor, in particular short circuits to a battery voltage or to ground or line interruptions of the inductive sensor used in a motor vehicle, for example a crankshaft sensor or camshaft sensor of an internal combustion engine or of a wheel sensor of an antilock brake system (ABS) or traction control system (TCS).
An inductive sensor is, as a rule, composed of a coil that is wound around a magnetized core. A signal transmitter element in the form of a gearwheel made of a ferromagnetic material that moves in synchronism with the crankshaft, the camshaft or a wheel is sensed by the inductive sensor. The edges of the gearwheel induce a characteristic voltage in the coil of the sensor as they move past it. The voltage is sensed by an evaluation electronic system which is connected downstream of the sensor and contains a comparator, and the voltage is shaped to form a digital output signal which is further processed in downstream control devices.
Basically, inductive sensors can be used wherever the position, rotational speed, or velocity of a movable signal transmitter element is to be sensed.
If such a sensor has symmetrical input wiring, in the event of a line interruption or short circuit it continues to supply an output signal whose profile does not permit reliable detection of the fault. However, this does not allow the strict requirements associated with prescribed on-board diagnosis to be met reliably and would result in incorrect diagnoses. In the course of development of electronic components for motor vehicle electronics, it has become apparent that there is a need to enable short circuits or line interruptions that may occur at any points in the sensor or in the sensor lines to be sensed, evaluated and displayed.
Published, Non-Prosecuted German Patent Application DE 28 41 289, corresponding to U.S. Pat. No. 4,344,032, discloses a method and a device for testing inductive sensors. A first signal that is proportional to the amplitude of the voltage generated by the pulse transmitter is formed, and at the same time a second signal which is proportional to the frequency of the pulses is formed. From the ratio of the first to the second signal, a third signal is formed which is compared with a predefined variable which is typical of the configuration of a pulse transmitter, the difference between the third signal and the typical variable being used as a quality criterion. The three signals are formed simultaneously by a coil as a low-pass filter that is connected to a shunt resistor between the terminals of the inductive signal transmitter, an evaluation unit being connected to the connecting point of the coil and the resistor. This method and the associated device serve the purpose of enabling the inductive pulse transmitter to be checked in the frequency range or rotational speed range that is of interest, particularly at low rotational speeds. In addition, the intention is to test whether sufficiently high signals are output in wheel sensors of antilock brake systems so that the position of the wheel sensor can also be tested with respect to the signal transmitter element. Sources of faults that occur essentially directly in the inductive sensor are detected and evaluated.
Published, Non-Prosecuted German Patent Application DE 195 26 435 A1 discloses a circuit configuration for detecting fault currents or leakage currents on the supply line of an electronic circuit when the supply voltage is switched off. Both a current source that is fed from an auxiliary voltage and a voltage divider which connects to ground are connected to the supply line, the potential which is obtained on the supply line when the supply voltage is switched off being determined and evaluated via the tap of the voltage divider by a potential monitor. When the voltage supply is switched on, it is not possible to use this circuit to detect either short circuits to the battery voltage or to the earth potential or line interruptions.
It is accordingly an object of the invention to provide a diagnostic device for detecting short circuits or line interruptions in an inductive sensor which overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which short circuits to ground or to the battery voltage can be reliably detected, evaluated and displayed not only when the sensor is stationary, but also in the operational state. In addition, line breaks in the sensor or its feeder lines in the entire frequency range of the sensor can also be detected and evaluated.
With the foregoing and other objects in view there is provided, in accordance with the invention, a diagnostic device for detecting short circuits and line interruptions in an inductive sensor formed of a coil wound around a magnetic core and generating a sensor output signal to be converted into a digital sensor signal for further processing in a control device of an evaluation electronic system, the diagnostic device containing:
a resistor for receiving a voltage reference;
a voltage divider to be connected between terminals of the inductive sensor and having a tap connected to the resistor for receiving the reference voltage via the resistor; and
an evaluation device connected to the voltage divider for receiving and conditioning a diagnostic signal tapped off at the tap resulting in a conditioned signal, the evaluation device comparing the conditioned signal with threshold values, a short circuit or a line interruption is detected if the threshold values are exceeded or undershot.
In accordance with an added feature of the invention, the voltage divider is a symmetrical voltage divider in which the resistors are of equal value.
In accordance to an additional feature of the invention, the evaluation device has an analog/digital converter receiving the diagnostic signal. The analog/digital converter samples the diagnostic signal in continuously successive measurement periods in one of a constant interval and chronologically unequal intervals that are longer or shorter than a period length of the sensor output signal. A largest sampled value Vmax, a smallest sampled value Vmin and their difference Vss is determined in each measurement period. The evaluation device further has a comparator unit connected to the analog/digital converter for comparing the largest sampled value Vmax, the smallest sampled value Vmin and the difference Vss with the threshold values. If an internal combustion engine is stationary and if Vmax=Vmin=Vdiag, where Vdiag is the diagnostic signal, a short circuit to a battery voltage is detected if Vdiag less than S1 or a short circuit to earth is detected if Vdiag less than S2. If the internal combustion engine is operating, the short circuit to the battery voltage, the short circuit to earth, or a line interruption in the sensor feeder lines are detected if Vss greater than S3, where S1, S2 and S3 are the threshold values.
In accordance with another feature of the invention, the diagnostic signal has a d.c. voltage component and the evaluation device, includes a high-pass filter for filtering out the d.c. voltage component of the diagnostic signal resulting in a filtered signal. A rectifier is connected to the high-pass filter and receives and rectifies the filtered signal resulting in a rectified signal. A capacitor is connected to the rectifier for receiving and smoothing the rectified signal resulting in a d.c. voltage signal. A comparator unit receives and compares the diagnostic signal with a first threshold value and a second threshold value of the threshold values. The comparator unit further receives and compares the d.c. voltage signal with a third threshold value of the threshold values. If an internal combustion engine is stationary and if Vmax=Vmin=Vdiag a short circuit to a battery voltage""is detected if Vdiag greater than S1 and a short circuit to earth is detected if Vdiag less than S2 and, if the internal combustion engine is operating, the short circuit to the battery voltage, the short circuit to the earth, or a line interruption in sensor feeder lines are detected if Vssxe2x80x2 greater than S3xe2x80x2, where S1 is the first threshold value, S2 is the second threshold value, S3xe2x80x2 is the third threshold value, Vdiag is the diagnostic signal, vssxe2x80x2 is the d.c. voltage signal, Vmax is a largest sampled value, and Vmin is a smallest sampled value.
In accordance with a concomitant feature of the invention, a display unit is provided and connected to the comparator unit for displaying the short circuits and the line interruptions detected in the comparator unit.
The diagnostic device according to the invention has the essential advantage that simple hardware components or software components are sufficient to detect and display the aforementioned faults reliably.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a diagnostic device for detecting short circuits or line interruptions in an inductive sensor, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.